1. Field of the Invention
This invention is directed to optically clear xerogel polymer compositions containing an interpenetrant. These compositions are characterized by the presence of hydroxyl functionalities which are blocked with a removable blocking group which, after removal of the blocking groups and hydration of the composition, will have a water content of at least 10 weight percent and preferably from about 35 to about 70 weight percent and a modulus of at least about 2 Mdyne/cm.sup.2.
This invention is further directed to methods for the preparation of optically clear hydrogel polymer compositions containing hydroxyl functionality and an interpenetrant.
2. State of the Art
Hydrogel polymer compositions and the use of these compositions in ophthalmic devices, especially contact lenses, are well known in the art. Such hydrogel polymer compositions are typically manufactured as copolymeric systems, optionally cross-linked which are formed in the xerogel state where they are hard materials. This xerogel, in the presence of water or other water containing solvent, hydrates and undergoes a change so that it attains the hydrogel state. Upon hydration, the resulting polymer composition contains water and, accordingly, becomes softer and more pliable as compared to the polymer composition prior to hydration. The degree of softness and pliability is related to the amount of water incorporated into the polymer composition after hydration. Additionally, contact lenses derived from polymer compositions having large amounts of water provide greater wearer comfort and higher oxygen permeability. Accordingly, the art has generally been directed to the incorporation of large amounts of water into such polymer compositions.
However, notwithstanding the desirability of incorporating large amounts of water into hydrogel polymer compositions, one problem typically encountered is that as the water contents increases, the structural rigidity of the polymer composition, as measured by its modulus, decreases and can reach a point where the structural rigidity is less than desirable. Accordingly, the hydrogel polymer composition is typically formulated to balance the need for a large water content and for a suitable modulus and the values achieved for both parameters is often compromised from ideal values.
In regard to the above, it is known in the art that an interpenetrant incorporated into a polymer composition increases the structural rigidity of the composition thereby providing a means to obtain a desired level of water content while retaining suitable structural rigidity.
However, a problem is encountered in the area of ophthalmic devices when a large amount of an interpenetrant, i.e., greater than about 1.5 weight percent (based on the dry weight of the polymer composition), is incorporated into a hydrogel polymer composition comprising hydroxyl groups. Specifically, it has been found that the use of such a large amount of interpenetrant in such hydrogel polymer compositions renders the resulting composition optically opaque. Without being limited to any theory, it is believed that the hydroxyl comprising polymer compositions have poor solubility for the interpenetrant and, accordingly, optical opacity for the resulting composition arises from phase separation of the interpenetrant from the polymer. In any event, optically opaque materials cannot be used in ophthalmic devices because optical clarity is an overriding criticality in these devices.
In one embodiment, the art has circumvented this problem by including large quantities of an organic solvent (e.g., about 80-95 weight percent or more), such as dimethyl sulfoxide (DMSO), with an interpenetrant chemically modified to include a reactive vinyl group. See, for example, European Patent Application Publication No. 0 456 611. The organic solvent acts to solubilize the interpenetrant as well as the monomer/polymer composition and the reactive vinyl group acts to covalently incorporate the interpenetrant into the polymer backbone during polymerization.
After polymerization, the resulting polymer is solvated (i.e., not a xerogel). The inclusion of large amounts of solvent in the polymers via such methods complicates the manufacturing process of ophthalmic devices from hydrogel materials because such materials are typically cast or formed in the xerogel state, and once solvated, become soft and pliable which makes machining the solvated materials difficult. Accordingly, the final shape and other physical characteristics of the polymeric article are preferably formed during the xerogel state, i.e., in the absence of significant amounts of any solvent. The inclusion of large amounts of solvent in the prior art methods for forming an optically clear polymer composition containing an interpenetrant will, however, preclude the formation of such a xerogel composition.
In view of the above, the art has heretofore been seeking, without success, an optically clear xerogel polymer composition comprising hydroxyl groups on the polymer and having incorporated therein at least about 1.5 weight percent of an interpenetrant.